Music to your ears? A compound that blocks both β- and γ-secretase might tone down production of amyloid-β (Aβ) and perhaps slow or prevent Alzheimer disease. In the June 30 Journal of Neuroscience, researchers describe how two already approved drugs raise the pH in endosomes just enough to put a damper on acid-loving β-secretase (BACE). Unexpectedly, the compounds also knock γ-secretase off its stride, irrespective of the pH effect. ARF first reported on this dual-secretase modulation when Stefan Lichtenthaler, University of Munich, Germany, presented preliminary findings at the 8th annual Eibsee Meeting on Cellular Mechanisms of Alzheimer Disease, January 2009 (see ARF related news story).
At Eibsee, Lichtenthaler described how bepridil, a calcium-channel blocker, inhibits BACE in cell culture with an IC50 or about 6 μM, and he hypothesized the pH effect might be the modus operandi. First author Stefan Mitterreiter and colleagues now report that bepridil, and another calcium-channel blocker, amiodarone, raise endosomal pH from 5.1 to 5.5. This may not sound like a huge increase, but at 4.5-4.8, the optimal pH range for BACE is narrow, making the enzyme exquisitely sensitive to small changes in acidity (see Shimizu et al., 2008). Furthermore, the pH change may be greater near the membrane, where BACE meets amyloid precursor protein (APP). Because both bepridil and amiodarone have lipophilic moieties connected to a basic amino acid, Mitterreiter and colleagues surmised that the compounds insinuate into cell membranes and that their protruding amino group raise the local solvent pH close to the membrane surface. In fact, a lipophilic, pH-sensitive fluorophore revealed that both compounds, at 20 μM, increase membrane-proximal pH by up to 0.6 pH units in lipid vesicles but have no effect on bulk pH.
If this is their mode of action, then the pH change is sufficient to inhibit BACE. When added to HEK293 cell cultures, both compounds dose-dependently blocked production of sAPPβ, the β-cleavage product of APP. Primary neurons from Tg2576 mice produced 60 percent less sAPPβ and about 30 percent less Aβ when treated with 5 μM bepridil. And when injected into the belly of guinea pigs at 50 mg/Kg, which equates to about 20 μM in plasma, bepridil also reduced plasma Aβ40 levels by half. The authors acknowledge that it is unclear whether the compound has any effect on BACE activity in the brain.
As reported at Eibsee, bepridil also works as an “inverse” γ-secretase modulator, meaning it blocks production of Aβ38, has little effect on Aβ40, and shifts sAPPβ cleavage more toward Aβ42. In the paper, Mitterreiter reports that amiodarone has the same profile. The γ-secretase modulation does not appear to be related to pH changes, however, as it occurred in cell-free γ-secretase assay. To confirm this, the researchers swapped the amiodarone basic group for an acidic one. The acidic version still modulated γ-secretase, albeit in a “straight” fashion, limiting Aβ42 production most. That observation raises the possibility of developing dual inhibitors that simultaneously block β- and γ-secretases.—Tom Fagan
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